diff -r c1bd4ffb5e47 src/devices/wifi/dcf-manager.cc
--- a/src/devices/wifi/dcf-manager.cc	Tue Sep 29 10:16:17 2009 -0700
+++ b/src/devices/wifi/dcf-manager.cc	Wed Sep 30 00:47:21 2009 +0400
@@ -244,6 +244,7 @@
     m_lastSwitchingDuration (MicroSeconds (0)), 
     m_rxing (false),
     m_slotTime (Seconds (0.0)),
+    m_slotTimeInvert (Scalar (1) / Seconds (10.0)),
     m_sifs (Seconds (0.0)),
     m_phyListener (0),
     m_lowListener (0)
@@ -274,6 +275,7 @@
 DcfManager::SetSlot (Time slotTime)
 {
   m_slotTime = slotTime;
+  m_slotTimeInvert = Scalar (1) / slotTime;
 }
 void 
 DcfManager::SetSifs (Time sifs)
@@ -519,7 +521,7 @@
       Time backoffStart = GetBackoffStartFor (state);
       if (backoffStart <= Simulator::Now ())
         {
-          Scalar nSlots = (Simulator::Now () - backoffStart) / m_slotTime;
+          Scalar nSlots = (Simulator::Now () - backoffStart) * m_slotTimeInvert;
           uint32_t nIntSlots = lrint (nSlots.GetDouble ());          
           uint32_t n = std::min (nIntSlots, state->GetBackoffSlots ());
           MY_DEBUG ("dcf " << k << " dec backoff slots=" << n);
diff -r c1bd4ffb5e47 src/devices/wifi/dcf-manager.h
--- a/src/devices/wifi/dcf-manager.h	Tue Sep 29 10:16:17 2009 -0700
+++ b/src/devices/wifi/dcf-manager.h	Wed Sep 30 00:47:21 2009 +0400
@@ -324,6 +324,7 @@
   Time m_eifsNoDifs;
   EventId m_accessTimeout;
   Time m_slotTime;
+  TimeInvert m_slotTimeInvert;
   Time m_sifs;
   class PhyListener *m_phyListener;
   class LowDcfListener *m_lowListener;